Techniques of forming e.g., logic MOS transistors using relatively low gate voltage and e.g., input/output MOS transistors using relatively high gate voltage on the same substrate have been developed (e.g., refer to JP-A-No. 11-67927). A relatively thin gate insulating film is used for MOS transistor with low gate voltage, whereas a relatively thick gate insulating film is used for MOS transistor with high gate voltage.